The genesis of Proterozoic massif-type 
anorthosites: Re-Os isotopic evidence

Henrik Schiellerup, NTNU, Norway

The source of the magma and geodynamic setting of massif-type anorthosites remain long-standing controversies in Precambrian geology. No actual consensus exists as to the nature of the parental magmas, or whether these magmas primarily originate in the Earth's mantle or lower crust.

The aim of our study has been to constrain the source and source variability among several intrusive units in the Rogaland province, including both anorthosites, anorthosite hosted sulphide deposits and mafic intrusions. Whereas conventional isotope tracers have generally failed to resolve mantle versus crustal source regions to massif-type anorthosites unambiguously, the Re-Os isotope system is uniquely suited to evaluate such source models. Extreme fractionation of Re from Os during mantle melting results in vastly differing mantle and crustal reservoirs, in terms of Os-isotope composition. At the same time, the Os-isotope composition of various mantle reservoirs remain relatively invariant in comparison to older crust.

Our Re-Os and Sm-Nd isotopic data yield isochrons that testify to a common heritage of anorthosites, mafic intrusions and sulphide deposits in Rogaland, and thus supports the notion of a common parental magma to account for the various intrusive units. The data are strongly in favour of an entirely lower-crustal origin of the parental magmas. Modelling implies that a mantle genesis would require crustal contaminants much older than what have currently been documented, extreme compositions or/and excessive amounts of assimilation.

References:

Schiellerup, H., Lambert, D.D., Prestvik, T., Robins, B., McBride, J.S. and Larsen, R. (2000). Re-Os isotopic evidence for a lower crustal origin of massif-type anorthistes. Nature 405: 781-784